Process for the production of hollow bodies from thermoplastic material

ABSTRACT

In that process for the production of hollow bodies from thermoplastic material by extrusion blow molding in which discontinuously extruded preforms are expanded in a blow molding mold, at least during that part of the filling phase which is typical of such a process, for an extrusion head, the distance covered by an ejection piston associated with a storage chamber and the speed at which the ejection piston is displaced within the storage chamber by the plasticised material are detected and compared by a central control and regulating device to the respective reference values. On the basis of the result of the comparison operation at least one operating parameter of the process, for example the delivery output of the extruder or the extent of the emptying strike movement of the ejection piston can then be adapted while still in the same working cycle in such a way that substantially the required preform volume is ejected.

The invention relates to a process for the production of hollow bodiesfrom thermoplastic material, in which process firstly preforms arediscontinuously produced and are then expanded under an internalincreased pressure in the mold cavity of a divided blow molding mold. Anextrusion blow molding machine provided for that purpose comprises anextrusion unit and a blow molding unit, the respective operatingprocedures of which must be matched to each other.

Such an extrusion unit has at least one continuously operating extruderfor plasticising the plastic material which is generally in granulateform, and an extrusion head in which there are disposed at least onestorage chamber for storage of the plasticised material and at least onemeans for emptying the storage chamber, said emptying means beingreciprocatable with a stroke-like movement between two limit positions.In that arrangement the storage chamber is generally annular and themeans for emptying the storage chamber is in the form of an annularpiston. Therefore, for the sake of simplicity, reference is primarilymade hereinafter to an annular piston, without that being intended toinvolve a limitation.

In the discontinuous formation of the preforms, it is possible to make adistinction between two operating phases in dependence on the strokemovement of the annular piston within the extrusion head during aworking cycle. In the one operating phase, the filling or storage phase,the storage chamber is filled with plasticised material, withdisplacement of the annular piston, until the annular piston reaches itslimit position at the end of the filling operation. In the followingoperating phase, being the ejection or emptying phase, the storedmaterial is ejected from the storage chamber under the effect of theannular piston which is moved by drive means, and urged towards theoutlet opening of the extrusion head, with the plasticised materialflowing through a communicating duct which is disposed between thestorage chamber and the outlet opening. During that second operatingphase the ejected material for the preform is predominantly formed bythe material being urged out of the storage chamber by the annularpiston. However also added thereto is a portion of material which isconveyed into the extrusion head by the continuously operating extruderduring the ejection stroke movement. The volume of the total amount ofmaterial which issues from the extrusion head is accordingly made up ofa portion by volume which was previously stored in the at least onestorage chamber, and a portion by volume which is conveyed into theextrusion head during the ejection period by the at least one extruder.The uninterrupted operation of the extruder for plasticising thematerial is desirable so that the operating condition thereof can bestabilised. The piston reaches its other limit position at the end ofthe emptying operation.

The preform which is formed during the ejection operation and whichgenerally hangs vertically at the outlet opening of the extrusion headis received by a blow molding mold which is at least divided into two.In that respect, at least a part of the central portion by volume of theoveral portion by volume which forms the preform is expanded under anincreased internal pressure within the mold cavity of the blow moldingmold. The end portions of the overall portion by volume are squeezed offas waste material by a squeezing-off operation in which the blow moldingmold portions are involved so that the waste material remains outsidethe mold cavity. After expansion of the central portion by volume, thehollow body formed is cooled until it is of sufficient stiffness so asto retain its shape upon being removed from the blow molding mold.

Accordingly the cycle time of a working cycle of such a blow moldingunit is essentially composed of a receiving time, an expansion time, acooling time and a removal time and movement times in respect of theblow molding mold portions, while possibly still further additionallyrequired periods have to be taken into account, which are known to anyman skilled in the art and which are therefore not discussed in furtherdetail herein. It is known that in particular the cooling time has aneffect on the duration of the overall working cycle. As the timesrequired for the other movement procedures involved in the blow moldingmold also cannot be readily freely selected, the cycle time of the blowmolding unit is usually taken as the cycle time of the entire extrusionblow molding machine.

That means that the cycle time for the working cycle of the associatedextrusion unit is also fixed as the endeavour is to provide for theproduction of a preform during a working cycle of the blow molding unit.In the case that two or more blow molding units are associated with ahead, the cycle time for the extrusion unit corresponds to acorresponding fraction of the cycle time of a blow molding machine. Itis also possible to provide a plurality of extruders in an extrusionunit, in which case that generally has an effect on a reduction in therequired delivery outputs of the individual extruders. Hereinafterhowever reference is primarily made to an extruder with which a blowmolding unit is associated, without that being intended to involve alimitation.

The production process takes place in a particularly advantageous mannerif the preform is ejected as precisely as possible when the blow moldingmold is ready to receive it. Operating procedures on the part of theblow molding unit or the extrusion unit, which depart from thatsituation, result in a reduction in the level of productivity andpossibly also result in the quality of the products being impaired.

In many cases, in the production of hollow bodies by means of extrusionblow molding, it is appropriate and possibly even necessary for the wallthickness or gauge of the preform to be influenced during the ejectionprocess, for example in such a way that the regions on the preform whichin the subsequent expansion thereof under an increased internal pressureare subjected to a great stretching effect are of greater wall thicknessthan those parts which are only less expanded. In that way it ispossible to produce hollow bodies which are adapted to the respectiverequirements involved and in particular hollow bodies of substantiallyuniform wall thickness. That is achieved by virtue of the fact that thewidth of the generally annular outlet opening of the extrusion head isenlarged or reduced in width in known manner for example by means of amovable internal portion. In that arrangement, to move the internalportion, use is made of a wall thickness programmer which, during theemptying phase, that is to say during the emptying stroke movementperformed by the annular piston, moves the internal portion of theoutlet opening into a predetermined position which corresponds to therespectively desired wall thickness, in dependence on the distancecovered by the annular piston. In that connection it is assumed that thearrangement generally provides a constant ejection speed on the part ofthe annular piston and a constant delivery output on the part of theextruder. A process for the production of hollow bodies fromthermplastic material by a blow molding process in which the preformsare discontinuously produced using a wall thickness programmer isdisclosed in German published specification (DE-AS) No 25 44 609.

Particularly when using wall thickness control means, it is necessary toensure that the preform is ejected from the extrusion unit, with thedesired volume, as otherwise the predetermined distribution of wallthickness is displaced along the longitudinal extent of the preformrelative to the mold cavity of the blow molding mold, thus resulting ina hollow body with the wrong distribution of wall thickness.

The working procedure of the extrusion unit is generally to be soadjusted to the working procedure of the blow molding unit that thevolume required for a preform is plasticised by the extruder within theperiod of a working cycle. It is only the delivery output of theextruder which usually operates continuously, that is determined on thebasis of that condition. In order to carry out the process described inthe opening part of this specification and in regard to design of theextrusion head required therefor, it is also necessary to define furtheroperating parameters for the working procedure, for example the volumeof the material to be stored, which is adjusted by the stroke movementof the annular piston within the storage chamber, and the ejection speedof the annular piston.

When adjusting those operating parameters, care has to be taken toensure that the total material issuing from the outlet opening does notjust correspond to the material which is collected in the storagechamber in the filling phase. On the contrary, as already mentioned, itis made up of the previously stored material and the material which isconveyed into the extrusion head during the ejection phase by theextruder. Accordingly the volume of the portion of material to becollected in the storage chamber is such that, together with the portionof material which is delivered by the extruder during the ejectionphase, it gives the volume which forms the preform.

Hereinafter the overall volume which forms the preform is referred to asthe gross volume which accordingly corresponds to the volume of materialof the expanded hollow body plus the volume of material constituting thewaste.

It is generally known that, due to the generally relatively narrowoutlet opening and the characteristics of the plastic material onissuing from that opening, it is not possible for the material dischargespeeds to be set at just any level, without flow markings or the likeoccurring on the surface of the preform under certain circumstances,with the result that the finished hollow body does not present therequired level of quality in respect of its surface nature. Accordingly,a speed range which is suitable for the respective plastic material witha given outlet opening is selected for the material discharge speed. Asthe required gross volume is to issue from the outlet opening during theejection phase, the time within which the ejection operation takes placecan be defined by means of the appropriate discharge speed.

Accordingly the cycle time of the extrusion unit can be divided into twosuccessive periods, the storage time and the ejection time. The workingprocedure of the associated blow molding unit also predetermines themoment in time at which the ejection operation can begin at theearliest, as it is only at that time that the divided blow molding moldis in its readiness position in which it can receive the preform. Thatis important for the reason that in many cases the closing unit of theblow molding mold is constantly disposed under the outlet opening of theextrusion head so that discharge of the plasticised material when theblow molding mold is not yet closed can result in serious disturbancesin the machine and/or disturbances in the operating procedure.

On the basis of those three operating parameters, namely the grossvolume, the cycle time and the ejection time, which are predeterminedfor the extrusion process having regard to the preform to be produced,the operating procedure of the blow molding unit and the properties ofthe thermoplastic material, it is possible to define the other operatingparameters of the ideal extrusion process.

The volume of the material to be stored is made up of the amount ofmaterial which is delivered to the extrusion head by the extruder duringthe storage time which corresponds to the difference between the cycletime and the ejection time. As the piston and accordingly the storagechamber are of a constant cross-section, that therefore establishes thelength of the stroke movement which the piston performs during thestorage phase by virtue of the displacement thereof under the effect ofthe plasticised material. The annular piston must cover that strokemovement distance during the ejection phase within the ejection time, ina direction towards the outlet opening, so that the ejection speedthereof can be ascertained from that condition.

In that connection the volume to be stored is smaller than the overallvolume issuing from the extrusion head; in that respect, if theextrusion process takes place in accordance with those referenceparameters, during the ejection phase it is just the amount of materialwhich corresponds to the difference between the volume required for apreform and the stored material, that is plasticised by the extruder.

The gross volume of the preform, which is referred to for the purposesof describing the invention, is the weight of the hollow body to beproduced together with the waste portions which do not take part in theexpansion process. As the density of the thermoplastic material changesin dependence on temperature and pressure, the same mass of plasticmaterial is of different volumes during the extrusion procedure. Thegross volume therefore represents an abstract parameter which can bechecked at the end of the production process by a check in respect ofweight of the overall amount of material ejected. The gross volume forthe purposes of describing the process is advantageous for the reasonthat the adjustable operating parameters of the extrusion process,namely the stroke movement of the piston and the speed of the piston,also represent geometrical parameters during the ejection phase. At anyevent there is a direct relationship between the required weight and theset gross volume as, if the overall preform is of excessively lowweight, it is to be assumed that the selected gross volume is too smalland vice-versa.

In operation of an extrusion unit, it must be borne in mind that thesystem comprising the extrusion head and the at least one extruder issubjected to influences which are unforeseeable and which cannot alwaysbe defined. Such influences involve inter alia fluctuations intemperature and deviations in the physical properties of the plasticmaterial to be processed, from the reference values thereof. Bothinfluences may result for example in fluctuations in the viscosity ofthe plastic material in the system, and they also have an effect inparticular on the efficiency of the extruder, that is to say the amountof material delivered per unit of time.

Not least due to the above-described influences, the extrusion blowmolding process, like all other technical processes, is subjected tofluctuations of generally greater or lesser magnitude and thusinaccuracies in regard to a predetermined reference or desired operatingstatus which is to be observed. Accordingly the properties of theproducts produced by such a process also exhibit fluctuations which inpart can result in serious reductions in quality. It is thereforenecessary to involve regulating devices which detect the deviations ofindividual items of equipment for carrying out the process involved,from a reference value which is to be observed, and produce suitablevariations in the regulated operating parameters in order to adapt themto the reference value.

Known regulating devices for the working procedure for discontinuousproduction of preforms detect for example the position of the annularpiston at the predetermined moment of time at which the ejectionoperation begins and at which the blow molding mold is ready to receivethe preform, and compare that actual position to a predeterminedreference position. Other regulating devices detect the moment in timeat which the reference volume to be stored is collected in the storagechamber, and compare that actual time to a set reference time. In theevent of possible deviations of the actual values from the referencevalues, the delivery output of the extruder is altered, with the aimthat in the next cycle the distance covered by the annular piston withinthe reference storage time should correspond to the reference value asaccurately as possible. A disadvantage in that respect is that it isonly in the next following working cycle that it is possible to checkwhether the desired result has actually been achieved, so that generallya plurality of cycles will be required in order to achieve apredetermined operating condition. The preforms which were producedbefore that time and thus also the finished hollow bodies willaccordingly not be of the desired quality.

It is desirable, and known from German laid-open application (DE-OS) No28 13 241 for the extrusion head to be of such a design configurationthat the maximum volume of the storage chamber is somewhat greater thanthe volume which must be stored in the production of preforms. Thatmeans that the stroke movement to be performed by the annular piston forthe production of a preform for a given hollow body is somewhat shorterthan the maximum possible stroke movement which is predetermined by thestorage chamber. In that connection, in known manner, the volume of theportion of material which is to be stored for the respective preform isadjusted by establishing, in the range of the maximum possible stroke,the limit positions of the stroke movement of the annular piston andthus its stroke which is therefore normally less than the maximumstroke. Accordingly it is possible to use the same head which representsa relatively expensive capital investment item, for different blowmolding molds for which different preforms with different gross volumesare required. In the case of a deviation in respect of the deliveryoutput of the extruder, that is to say if the volume to be stored isplasticised too early or too late, it is possible with such an extrusionhead, at the reference or desired beginning of the ejection phase, forthe piston generally to perform a stroke movement of the adjustedlength, as the extrusion head has a compensating volume from whichadditional material can be taken if the delivery output is excessivelylow or in which excess material can be stored if the delivery output isexcessively high.

It has been found that the known processes suffer from the disadvantagethat the filling condition of the storage chamber is detected only onceduring a working procedure, without in that respect it being possible toascertain the way in which a deviation which has possibly occurred cameabout. If for example the annular piston has not reached its referenceposition because for just a brief moment the extruder suffered from adrop in output, for example as a result of the granulate materialfailing to slide down in the proper manner in the associated fillinghopper, then with the known regulating systems the delivery output ofthe extruder will be increased although the delivery output againcorresponds to the reference value. The result of that is that in thenext cycle the annular piston reaches its reference position much tooearly so that a plurality of interventions are required in order tostabilise the procedure. In addition, in known processes, the amount ofmaterial conveyed into the extrusion head during the ejection phase isnot sufficiently taken into consideration so that the procedure does notadequately comply with the rising level of requirements in terms ofaccuracy of the hollow bodies to be produced.

The invention is accordingly based on the problem of providing a processfor the production of hollow bodies of thermoplastic material by anextrusion blow molding procedure of the kind set forth in the openingpart of this specification, which eliminates or at least markedlyreduces the above-indicated disadvantages. In particular the inventionseeks to improve the level of productivity of such machines so that asfew hollow bodies as possible, which do not comply with the qualityrequirements, are produced. Furthermore the invention seeks to providethat the process according to the invention simplifies the setting-upoperation, that is to say the operation of preparing a machine for theproduction of a given hollow body.

As a solution, the invention proposes that the operating conditions ofthe individual assemblies of an extrusion blow molding machine aredetected and related to each other in a control and regulating device.Starting from the predetermined operating parameters comprising grossvolume, cycle time and ejection time, the control and regulating deviceascertains the reference values, which are required for a working cycle,in respect of the values which influence the extrusion procedure, namelythe reference stroke movement for the annular piston, the referencedelivery output of the extruder and the reference speed of the annularpiston during an ejection phase, in accordance with the introductorycomments above.

In the event of a deviation occurring, depending on the magnitudethereof, at least one operating parameter and/or a preset value for anoperating parameter of the subsequent ejection process is varied, duringthe same working cycle, in such a way that substantially thepredetermined gross volume of the preform is ejected.

For that purpose, at least once during the filling operation, thedistance covered by the annular piston and the speed at which theannular piston is displaced by the plasticised material in the storagephase are detected and compared to associated reference values withinthe central control and regulating unit. In that situation, the speed ofthe annular piston during the filling operation is taken as ameasurement in respect of the delivery output of the extruder. In thatrespect it will be desirable for the speed and the distance covered bythe annular piston to be continuously detected in order to increase thelevel of accuracy, a plurality of times during the filling phase or atleast over a part of the filling phase.

That procedure according to the invention, using simple means, providesfor monitoring the procedure in respect of time of the working cycle ofthe extrusion unit and the delivery output of the at least one extruderwhich are reproduced by the distance covered and the speed of theannular piston, at least during a given period of the filling phase. Anydeviations from the respective reference or desired values are thereforedetected in good time so that it is possible to provide for earlyregulating intervention in relation to corresponding operatingparameters.

Detecting the operating conditions in that way means that it is possiblein an advantageous fashion to optimise the extrusion procedure so thatin the same working cycle in which the deviation was detected, it ispossible to intervene in the operating parameters in order still toproduce a preform of the prescribed gross volume. In that respect,intervention in that way to adjust operating parameters shouldadvantageously occur only up to a moment in time and in a manner suchthat it is possible for the effects of intervention still to bemonitored in the same working cycle. In particular adaptation of theoperating parameters will amount to ejection of a preform which is ofthe required gross volume with the greatest possible degree of accuracy,irrespective of the nature of a deviation and the moment in time atwhich it occurs, with the cycle time being unaffected or only slightlyaffected. That ensures a uniform degree of quality of the hollow body,with a high level of productivity.

In order to adapt required interventions in relation to operatingparameters, it is possible to deal with not only the delivery output ofthe extruder but for example also the length of the stroke movement tobe performed by the annular piston or the ejection speed of the annularpiston in the subsequent ejection phase. The last-mentionedinterventions afford the advantage that, in contrast to intervening inthe extruder system which reacts relatively slowly, they have an effectdirectly on the respective ejection process, virtually without any timedelay.

Another advantage is that the delivery output of the extruder isdetected directly so that it is possible to recognise and maintain astable operating condition thereof, which is generally desired. Thatavoids frequent interventions in relation to the operating parameters ofthe extruder so that the extruder can rapidly assume an advantageousmode of operation. In that respect it may be desirable for a variationin the delivery output of the extruder to be effected only when itexceeds an admissible deviation, over a given period of time. However,in the case of small deviations or short-term fluctuations in thedelivery output of the extruder, which would also cause the ejectedgross volume to deviate from the reference value thereof, it shouldoptionally also be possible to intervene in relation to other operatingparameters, by means of the control and regulating device according tothe invention, so that nonetheless the required gross volume is ejectedwith the maximum possible degree of accuracy.

In that respect, in relation to small deviations or short-termfluctuations in the delivery output of the extruder or in relation todeviations which are found only just before the beginning of theejection phase so that the effects of a change in the delivery output ofthe extruder can no longer be checked, the procedure may advantageouslyinvolve providing for an intervention to change the extent of theemptying stroke movement of the annular piston and/or the speed thereofduring the ejection operation, in such a way that the required grossvolume is produced. As those operating parameters can generally be resetquickly, it is particularly desirable in accordance with the inventionfor the distance covered by the annular piston and the speed thereof tobe detected at least at the end of the filling phase or just beforesame, as it is to be assumed that the deviations in the delivery outputof the extruder continue during the ejection phase and thus, using thosedetected values, it is possible to provide information about the furtherprogress of the extrusion procedure, by reference to which the otheroperating parameters can be altered in order to produce a gross volumewith the maximum degree of accuracy.

Thus it is possible for example, if the delivery output of the extruderis excessively low, to eject a correspondingly larger amount of materialfrom the storage chamber, whereas if the delivery output of the extruderis excessively high, it is possible to eject a correspondingly smalleramount of material from the storage chamber. That can be effected at anyevent when, as already mentioned, the storage chamber is of a maximumvolume which is greater than the reference volume to be stored forconstituting a preform, in which case the reference length of the strokemovement of the annular piston is then also shorter than the maximumpossible piston travel. That configuration affords the possibility, ifthe delivery output of the extruder is excessively high, of reducing thelength of the piston stroke movement relative to the reference value inrespect thereof, so that a smaller amount of material than the referenceamount to be stored is also ejected from the storage chamber by theannular piston. In that case, that smaller amount of material, togetherwith the amount of material which is conveyed into the extrusion head bythe extruder during the ejection phase and which is now greater byvirtue of the higher delivery output of the extruder, provides theamount of material required for a preform. The result of that is that apart of the stored portion of material still remains in the storagechamber at least during the subsequent filling phase.

Conversely, if the delivery output of the extruder is excessively low,material for forming the preform is additionally ejected from the supplyof material in the storage chamber of the extrusion head by acorresponding increase in the extent of the emptying stroke movement ofthe annular piston. That additional material is of such a quantity thatthe overall amount of material ejected from the storage chamber,together with the amount of material which is plasticised by theextruder during the ejection phase and which is now reduced by virtue ofthe excessively low delivery output of the extruder, gives the amount ofmaterial required for a preform.

It is important in that respect that, when there are deviations in thedelivery output of the extruder from the desired or reference deliveryoutput thereof, not only is the position of the reference strokemovement of the annular piston displaced within the storage chamberwhich is of larger size, and the annular piston performs only thatdesired or reference stroke movement, but in addition the length of thestroke movement is also changed in order to compensate for thedeviations in the delivery output of the extruder during the ejectionphase.

The increase or reduction in length of the piston stroke movement, withthe speed of the piston remaining unchanged, during the ejectionoperation, results in a variation in the ejection time. However, by acorresponding variation in the ejection speed it is possible to providethat the reference ejection time is maintained as accurately aspossible. Intervening in that way in relation to the length of thestroke movement on the one hand and the speed of the annular pistonduring the ejection operation on the other hand means that it ispossible to maintain both the required gross volume for a preform andalso the cycle time. In addition, as the reference volume for thepreform issues from the outlet opening within the prescribed referenceejection time, the discharge speed which is advantageous from the pointof view of the thermoplastic material is achieved.

If, under the specified conditions, the speed of the annular piston, atwhich the emptying stroke movement is performed, is not changed and thusthe period of time required for performing a shorter or a longer strokemovement is shorter or longer respectively than the reference time, withthe delivery output of the extruder being detected, that must alsoresult in a corresponding additional change in the emptying strokemovement so that the amount of material which is plasticised by theextruder in that time difference and which accordingly is greater orsmaller than the amount of material plasticised under the desired orreference conditions is also compensated. That procedure admittedlyresults in a change in the cycle time of the extrusion system, whichhowever is so slight that it can normally be tolerated, having regard tothe small proportion of material which is supplied by the extruderduring the emptying phase.

In the case of a discontinuously operating extrusion unit which has awall thickness control means for influencing the wall thickness of thepreform, in general the distance covered by the annular piston, duringthe ejection operation, is taken as a control signal for adjusting themeans which influence the wall thickness of the preform. Frequently, theprocedure is such that the set stroke movement is divided into aplurality of portions of equal size so that each portion of the strokemovement corresponds to a given portion by volume which issues from theextrusion head. When the annular piston is moving at a constant ejectionspeed, that portion by volume is made up of the stored volumecorresponding to the portion of the stroke movement, and thecorresponding fraction of the volume of material plasticised by theextruder during the ejection period.

So that each portion of the respective stroke movement of the pistonagain corresponds to a portion by volume of the same size, it isnecessary for the wall thickness program to be respectively based on thelength of the piston stroke movement which is altered by theintervention operations in accordance with the invention, and for thatlength to be divided into the same number of portions of the pistonstroke movement. That ensures that associated with each point in theprogram of the wall thickness control means there is once again adischarged volume of the same size, so that there is no distortion interms of the distribution of the wall thickness of the preform.

A corresponding point applies if the wall thickness control effect takesplace in dependence on the ejection time. In that case the respectivelyaltered time of the ejection phase would have to be based on the wallthickness program. However that time-dependent control of wall thicknessis not conventional practice in relation to the discontinuous productionof preforms.

By virtue of the presence of a central control and regulating unit whichis provided with a suitable computer, it is possible to monitor theentire operating procedure of the extrusion system; the computercomputes from the detected operating data the respective changes whicharise out of the respective deviations from the reference condition andwhich are required in order to obtain the desired operational result. Inparticular that is achieved by monitoring the operating condition of theextruder during the filling phase, whereby deviations can be detected ingood time and suitable steps can be initiated. The changes may serve toeliminate the respective deviation from the operating condition, forexample in regard to the delivery output of the extruder, as quickly aspossible and as extensively as possible. However it is also possible tooperate in such a way that the deviation is compensated by a change inanother operating parameter, for example by altering the length of thestroke movement of the annular piston associated with the storagechamber, and/or the speed of the annular piston, during the ejectionphase.

The way in which certain intervention operations in accordance with theinvention can be effected is described hereinafter with reference to thedrawings and in particular with reference to the volume-time graphs(V-t-graph) of the working cycle of a discontinuously operatingextrusion unit. In the drawings:

FIG. 1 is a diagrammatic view of an embodiment of an apparatus forcarrying out the process,

FIG. 2 is a V-t-graph of an ideal working cycle,

FIG. 3 is a V-t-graph with a deviation from the ideal working cycle,

FIG. 4 is a V-t-graph with another deviation from the ideal workingcycle,

FIGS. 5-8 are each a V-t-graph in which the effects of differentintervention possibilities according to the invention are shown,

FIG. 8A shows a portion from FIG. 8 on an enlarged scale, and

FIGS. 9-10 are each a V-t-graph showing the effects of furtherintervention possibilities in accordance with the invention.

The apparatus which is shown diagrammatically in FIG. 1 of the drawing,for the production of hollow bodies from thermoplastic material by meansof extrusion blow molding, comprises an extrusion unit 2 and a blowmolding unit 4. The extrusion head 9 of the extrusion unit 2substantially comprises a housing 15, a stationary bar or mandrel 16, acore 17 which is arranged displaceably coaxially therein, and a piston19 of annular configuration which is axially movable in a storagechamber 18. At its lower end 14 the core 17 represents the innerboundary of an outlet opening 20 which is in the form of an annular gap.In the other end region the core 17 is provided with a piston 21 guidedwithin a cylinder 22. The feed and discharge lines for the generallyhydraulic operating medium are identified by references 23 and 24. Thepiston 19 is connected to the piston rod 26 of a piston 27 by means ofrods 25 or the like which are arranged distributed around the peripheryof the piston 19. The piston 27 is arranged slidably in a cylinder 28.Feed and discharge lines 29 and 30 are provided for the normallyhydraulic operating medium.

Disposed upstream of the extrusion head 9 is an extruder 10, the screw11 of which is driven by a motor 12. The thermoplastic material which isgenerally in granulate form is fed to the extruder 10 by way of a hopper13. The extruder 10 opens into an intake opening 37 of the housing 15 ofthe extrusion head 9. In that arrangement the plasticised material isput into an annular cross-sectional shape by two mutually oppositelydisposed, downwardly enlarging ducts 32 and flows into the storagechamber 18 by way of an intermediate duct 33 which is formed by themandrel 16 and the movable annular piston 19.

Arranged beneath the extrusion head 9 is the blow molding unit 4 havinga blow molding mold 34 comprising two mold halves 34a and 34b.Associated with the blow molding mold 34 is a mandrel 35 which serves tosupply a pressure medium, for example compressed air, by means of whichthe preform 31 is expanded, to correspond to the contour formed by themold cavity of the blow molding mold 34 when closed. The pressure mediumis supplied by way of a bore 36 in the mandrel 35. To provide for themovement of the mold halves, associated with the mold half 34b is adrive piston 49 which is slidable in a cylinder 48. Knownsynchronisation means provide that the blow molding mold halves 34a and34b can move uniformly towards the preform 31. The synchronisation meansare not shown for the sake of clarity of the drawing, especially as theyare familiar to any man skilled in the art. It is also conventionalpractice for each blow molding mold half 34a and 34b to have its owndrive means 48, 49 associated therewith.

For the purpose of carrying out the process according to the invention,there is provided a control and regulating device, the central computingunit of which is identified by reference numeral 66. In that unit, whichcan be for example a microcomputer, the detected operating conditionsare evaluated or assessed and any adaptations in respect of theoperating parameters of the extrusion and blow molding unit areprescribed.

In order to detect the distance covered by the annular piston 19, theassembly has a position sensor 64 with a position indicator 39 which canbe for example in the form of a potentiometer. The position sensor 64 isconnected by way the line 56 to the central control and regulatingdevice 66 in which the actual position of the annular piston 19 iscompared to the reference position thereof, and evaluated. Theinstantaneous speed of the annular piston 19 is ascertained from theposition signal 56 in the conversion unit 68, and inputted by way of theline 69 to the central unit 66 in which the actual speed is compared tothe predetermined reference speed both during the filling phase and alsoduring the ejection phase. In accordance with the introductorydiscussion, those operating parameters represent the essential controlparameters of the extrusion procedure as the progress in respect of timeof the procedure can be ascertained by means thereof. The speed of theannular piston 19 during the filling phase is thus taken as ameasurement in respect of the delivery output of the extruder 10, thedrive motor 12 of which is connected to a regulating device 80 which inturn is connected by way of the line 81 to the central unit 66.

For the purposes of influencing the wall thickness of the preform, theextrusion unit 2 has a movable nozzle core 17 whose movements areregulated during the ejection operation by a programmer 53. The core 17is moved in dependence on the stroke movement of the annular piston 19,in accordance with the program represented by the curve 54. For thatpurpose the assembly has a valve 63 which co-operates with the feed anddischarge lines 23 and 24 of the piston-cylinder unit 21, 22. Theregulator 60 compares the actual position of the core 17, which isdetected by way of the position indicator 57 and the position sensor 58,to the reference position thereof which is prescribed by the wallthickness program, by way of the line 61. The stroke movement of theannular piston is inputted to the wall thickness programmer 53 by theposition sensor 64 by way of the line 56.

For the purposes of checking the prescribed final length of the preform31, the assembly may have for example at least one light barrierarrangement 38 in order for example to be able to establish whether thepreform has suffered from an increase or a reduction in its length, as aresult of the properties inherent in the plastic material. The at leastone light barrier arrangement is connected by way of a line 52 with aregulating device 55 and the wall thickness programmer 54 whereby forexample it is possible to produce an adjustment in the wall thicknessprogram for the purposes of influencing the ejected length of thepreform 31 without the gross volume of the preform 31 being varied. Thewall thickness programmer 54 and the central control and regulating unitare connected together by means of the lines 76 and 77 so that theprogram procedures can be adapted to each other in the event of possiblemutual influences, for example by virtue of a variation in the strokemovement to be performed by the annular piston 19.

During the ejection operation the reference ejection speed of theannular piston 19 is set by way of a valve 87 which co-operates with thepiston-cylinder unit 27, 28 of the drive means for the annular piston19. For that purpose the valve 87 is connected to a control unit 89which receives its input signal from the central unit 66 by way of theline 88. In that arrangement the speed of the annular piston 19 isdetected by the position sensor 64 and inputted to the central controlunit 66 by way of the conversion unit 68. It is also possible however toprovide a separate speed measuring device.

The length of the stroke movement to be performed by the annular piston19, which determines the volume to be ejected in regard to the materialstored within the storage chamber 18, can be established by theadjusting device 85 which, in dependence on a signal outputted by thecentral control and computing unit 66 by way of the line 83, sets thestroke travel of the annular piston 19 and the resulting limit positions40 and 41 in respect of the piston stroke movement.

The movements of the blow molding unit 4 are monitored and set by acontrol unit 70 which prescribes the cycle time, the cooling time andother necessary times of the blow molding unit. In that connection, thesignals for controlling the movement are supplied by way of a line 65 tothe control device 44 which is connected to a valve 45 for the feed anddischarge lines 46, 47 of the associated piston-cylinder unit 49, 48.The respective position of the blow molding mold is inputted to thecontrol unit 70 by way of a position sensor 73 which can be in the formof a potentiometer.

The control unit 70 co-operates by way of the lines 71 and 72 with thecentral regulating and control unit 66 for the extrusion process. Inparticular, the cycle time to be observed and the moment in time atwhich the blow molding mold 34 is ready to receive a preform are presetby the control unit 70 by way of the line 71 for the central control andregulating unit 66.

For the purposes of monitoring the weight of the ejected preform, thearrangement includes a weighing device 91 with which the expanded hollowbody with the waste or flash portions thereon can be weighed. Theweighing device 91 is connected to a comparison device 93 in which theactual weight is compared to the reference or desired weight which isprescribed by the presetting unit 95. The weight monitoring operationmay be effected for example in relation to each preform produced,although it may also be sufficient to measure only the weight of onepreform out of a plurality of preforms which have been produced insuccessive working cycles. The reference value in respect of the preformweight is inputted to the central control and regulating device 66,after conversion in a conversion unit 97 into a gross volume. Thecomparison device 93 is connected to the conversion unit 97 by way of aline 96 so that, in the event of deviations in the weight of thepreform, the conversion can be corrected. The conversion operation takesaccount of the density of the respective plastic material, without thevalue characterising the density of the material having to be known interms of an absolute parameter.

FIG. 2 shows the ideal configuration of a working cycle of an extrusionunit of the kind described in the introductory part hereof. The ordinateidentified by V represents the plasticised volume of the plasticmaterial. The various portions in respect of time of the workingprocedure of an extrusion process are plotted on the abscissa which isidentified by t. By virtue of the fixed geometry of the storage chamberand the annular piston within the extrusion head, it is possible for astroke movement produced by the annular piston to be directly associatedwith a volume so that, with a suitable choice of scale, the strokemovement of the annular piston can also be read off on the ordinate.Accordingly the curve configuration corresponds to the movements of theannular piston, which are used as the decisive monitoring parameter inrespect of the entire working cycle. The advantage of representing thevolume can be considered to lie in the fact that, beside the movement ofthe piston, the volume of the material extruded during the ejectionphase as well as the delivery output of the extruder are also shown.

The boundary or limit conditions for the extrusion procedure are definedby the required gross volume V_(B) (distance EC) for a preform on theone hand and the time t_(Z) (distance AE) for a working cycle on theother hand.

In accordance with the introductory discussion herein, an advantageousdischarge speed for the plastic material from the outlet opening duringthe ejection operation can be determined from the identifying parametersof the extrusion head and the plastic material. That, with the overallvolume V_(B) to be ejected, gives the ejection time t_(A) (distance DE)as a further prescribing parameter. All further reference values for theextrusion procedure can be ascertained with those prescribing parametersV_(B), t_(Z) and t_(A).

The delivery output of the continuously operating extruder can be seenfrom the gradient of the line AC in FIG. 2 as the gross volume V_(B) fora preform must be plasticised by the extruder within the time of aworking cycle t_(Z). Both the reference speed and also the referenceposition of the annular piston during the storage phase can be deducedfrom the configuration of the line AB. Thus, during the storage phase,the delivery output of the extruder can be monitored continuously or atgiven moments in time, by detecting the speed of and/or the distancecovered by the annular piston. In the ejection phase (distance DE) onthe other hand the delivery output of the extruder cannot be ascertainedby means of the speed and/or position of the annular piston.

The portion of the overall volume V_(B) which must be collected in thestorage chamber during the filling phase can be calculated from thecycle time t_(Z) and the ejection time t_(A). The ratio of the volumeV_(S) to be stored (distance DB) to the gross volume V_(B) (distance EC)corresponds to the ratio of the storage time t_(S) (distance AB) whichresults from the difference between the cycle time t_(Z) and theejection time t_(A), relative to the cycle time t_(Z) (distance AE).Accordingly the following relationship applies: ##EQU1##

In that situation the distance DE also corresponds to the referencestroke movement of the annular piston. The reference speed of and thedistance to be covered by the annular piston during the ejection phasecan be deduced from the configuration of the line BE.

As generally the required gross volume of a preform is determined underother conditions, in particular in regard to the pressure obtaining, thecompressibility of the plastic material means that it is necessary totake account of the pressure conditions obtaining in the storagechamber, the parameters of the material, and other influencingparameters, in applying the foregoing relationship concerning therespective volume of the material used. As however those influencingparameters are primarily related to each other in a more or lessconstant relationship, the difference resulting therefrom, between thegross volume of a given mass in the extrusion head on the one hand andthe gross volume of that mass in the finished article on the other handremain disregarded in the discussion in principle which is set forthherein.

As however that difference cannot be satisfactorily determined in apractical context, the overall weight of the preform is taken as apreset value for the gross volume V_(B), by the weight being convertedwith a given conversion factor into the gross volume. The weight of theoverall material to be ejected can be ascertained after a working cycleby weighing the expanded hollow body, together with the waste portionsof material; if the weight is excessively high, the gross volume to beused is reduced while if the weight is excessively low, the gross volumeto be used is increased, it being presupposed that the extrusion processproduced the respective underlying gross volume in question.

It can further be seen from FIG. 2 that the preform which is produced ina working cycle, of the gross volume V_(B), is made up of the portion ofmaterial V_(S) (distance DB or EF) which is collected in the storagechamber during the storage time t_(S), and the portion of material V_(E)(distance FC) which is conveyed into the extrusion head by the at leastone extruder during the ejection phase.

It can be seen from the graph and the mode of operation of suchextrusion units, which was described above, that the gross volume, cycletime and ejection time represent presetting or prescribed parameterswhich, in order to carry out correction operations, cannot be varied orcan be varied only within narrow limits. Correction operations, during aworking cycle, can concern the delivery output of the extruder, thelength of the stroke movement of the annular piston and the speedthereof during the ejection operation.

FIG. 3 shows a deviation from the delivery output of the extruder.Hereinafter operating conditions which deviate from the referenceconfiguration are identified with ' or ". In this example, a smalleramount of material per unit of time is plasticised by the extruder. Theresult of that is that the set volume V_(S) of the material to be storedwill also be attained at a later moment in time D'. At that later momentin time the material is displaced from the storage chamber at the setspeed of the annular piston so that the reference ejection time t_(A) ismaintained, although with a time shift (t_(A) ') as the stroke movementto be performed by the annular piston and the ejection speed thereofremain unchanged. The result of that is that, due to the lower deliveryoutput of the extruder during the ejection phase a smaller volume ofmaterial V_(E) ' (distance F'C') is conveyed into the extrusion head andaccordingly the ejected volume V_(B) ' is smaller than the referencevolume V_(B) of a preform. In addition, that will result in a longercycle time t_(Z) ' (distance AE').

In order to avoid misunderstanding, it will be pointed out at this stagethat the illustrated V-t-graphs only reproduce the qualitativeconfiguration of an extrusion procedure. Therefore, on the basis of theselected divisions of the axes of the co-ordinate system, the deviationsin volume may be very small and may thus appear insignificant. It isapparent however that even just small deviations in volume in respect ofthe ejected material may give rise to variations in the length of thepreform which for example may considerably displace the distribution interms of wall thickness, with respect to the longitudinal extent of thepreform, so that the finished hollow body does not exhibit the desireddistribution of wall thickness. The modes of operation involved are onlyto be described in principle, by reference to the illustrated examples.

Known apparatuses generally ascertain either the time difference(distance DD') between the reference time and the actual time at whichthe material to be stored is collected in the storage chamber, or, atthe reference moment in time D at which the stored material is to beejected, the position B" of the annular piston, and compare same to areference position B. On the basis of the result of the comparisonoperation, intervention is initiated, which in this case causes anincrease in the delivery output of the extruder. In that connection,such intervention will only be able generally to produce its effect inthe following working cycle, due to the slow reaction on the part of theextruder, so that at least in this cycle a preform is produced, whichdoes not comply with the desired requirements. There is also thedisadvantage that the effect of the intervention can be monitored at theearliest at the end of the storage phase in the next cycle.

FIG. 4 shows a failure in delivery output of the extruder, which lastsonly for a short time and after which the extruder regains its referencedelivery output. Such a variation in the delivery output of the extrudermay be caused for example by the plastic material in granulate formsuffering from a delay in slipping further down in the filling hoppertowards the extruder. As shown, the volume to be stored will first bestored in the storage chamber at a later moment in time D'. As in thiscase the delivery output of the extruder again corresponds to thereference value after just a short period of time, and thus the lineA'C' extends parallel to the line AC, the required volume V_(E) isconveyed into the extrusion head by the extruder during the ejectionphase (distance D'E'), so that the required gross volume is in factejected, although at a later moment in time (E').

However, in this case also known regulating devices ascertain either thedifference between the reference time and the actual time (distance DD')at which the volume of material to be stored is in fact stored, or thedifference between the reference position of the annular piston and itsactual position (distance BB") in which it is disposed after thereference storage time D. The result of that is that in this examplealso the delivery output of the extruder is altered by a regulatingintervention, although it has returned to its reference value againafter only a brief deviation, with the result that the next workingcycle involves conditions which constitute a deviation from thereference operating condition and which result in a defective preform.

The examples in FIGS. 3 and 4 show that extrusion processes with knownregulating and control devices which only ascertain the operatingcondition once per cycle, at least within the working cycle in which thedeviation has occurred, provide a preform which is produced in a cycletime which deviates from the reference cycle time, and that preform ispossibly of a gross volume which deviates from the reference value. Inaddition, such regulating and control devices provide interventionoperations which result in a change in an operating parameter at theearliest in the next working cycle so that the effects thereof can onlybe ascertained in that cycle. Accordingly, it may happen thatintervention operations were no longer required, at the moment in timeat which they were initiated. Erroneous interventions in that way in theextrusion procedure are in particular to be attributed to the fact that,throughout the entire storage phase which is generally longer than theejection phase, and which in many cases can even last for up to 90% ofthe cycle time, none of the decisive operating parameters and inparticular the delivery output of the extruder and the position of theannular piston are ascertained, evaluated and possibly altered.Generally therefore a plurality of working cycles are required in orderto compensate for fluctuations which have occurred.

A regulating and control device according to the invention seeks toavoid the disadvantages of the known regulating devices, in that atleast during a portion of the filling phase the speed of the annularpiston is detected as a measurement in respect of the delivery outputand/or the position of the piston and evaluated by the centralregulating and control unit. Depending on the moment in time and theextent of any deviations from the reference condition, interventionoperations are then initiated so that the required gross volume for apreform is ejected, with the greatest possible degree of accuracy. Theinvention further seeks to provide that the cycle time of the associatedblow molding unit is maintained and observed. The procedure will now bedescribed with reference to the following Figures, with the idealconfiguration of FIG. 2 also being shown for comparison purposes.

FIG. 5 represents movements of the annular piston which can occur whenthe delivery output of the extruder is regulated by a regulating andcontrol device according to the invention. In that respect, the speed ofthe annular piston and the positions thereof are detected a plurality oftimes during the filling phase and evaluated in the control andregulating device, with the result that, in the event of possibledeviations in respect of the extruder delivery output detected in thatway, the delivery output is corrected. It is apparent that detecting theactual values in that way and comparing them to the reference valuesmeans that intervention operations can be carried out, which are stilleffective in the same working cycle and the effect of which can still bemonitored therein. The result of that is that the end of the storagephase can be attained at the correct moment in time D, while thedelivery output of the extruder which essentially corresponds to thereference value can also be set. As now any fluctuations in the deliveryoutput of the extruder can only take an effect in the ejection phase andaccordingly only in respect of the volume V_(E) which is extruded duringthat period of time, in general the required gross volume V_(B) can beejected with a high degree of accuracy. At any event fluctuations whichoccur in the emptying phase are not of very great consequence in termsof quantity, and can therefore be tolerated in many cases.

It is known that a stable operating condition for an extruder depends ona number of influences so that generally even minor variations due toexternal interventions can result in compensating procedures occurringin the longer term. The endeavour will therefore be to avoid frequentinterventions in regard to the operation condition of the extruder.Accordingly in many cases it may possibly be disadvantageous to regulatean extrusion process in such a way as to give a working procedure asshown in FIG. 5.

On the contrary it will often be desirable for interventions in theoperating condition of the extruder to be initiated only when thedeviations of the delivery output exceed a predetermined tolerancerange. That may be the case for example when the extrusion unit isstarted up again after a break in operation, so that a stable operatingcondition in respect of all influencing parameters which have an effecton the delivery output of the extruder must first be attained. Thedeviations in the extrusion process, which occur until a stableoperating condition is attained, can be compensated by varying otheroperating parameters. That is possible for the reason that monitoringthe speed of the annular piston and the position thereof by the centralcontrol and regulating unit can provide information regarding the extentof the deviation during the on-going procedure and at the end of theextrusion process, and from that it is possible to derive interventionoperations in relation to other operating parameters in order to ejectthe gross volume required for a preform. In that respect it is assumedhereinafter that the deviations in delivery output, which are set in thefilling phase, continue in the ejection phase. Deviations which onlyoccur in the ejection phase cannot be ascertained and accordingly cannotbe evaluated.

FIG. 6, proceeding on the basis of a delivery output on the part of theextruder which is lower than the reference or desired delivery output,shows the effects of intervention operations which result in variationsin the volume of the material to be stored and/or the speed of theannular piston during the ejection operation so that substantially therequired gross volume is ejected. In regard to the first possibilitywhich is identified by ' in FIG. 6, the volume of the material V_(S) 'to be stored is increased (distance D'B') in such a way that, during theejection process and with the set reference ejection speed for theannular piston, the material V_(E) ' conveyed into the extrusion head bythe extruder (distance V'C'), together with the material V_(S) ' storedin that working cycle, gives the required gross volume V_(B). In thatsituation, there will be both a different moment in time for thecommencement of ejection (D') and also a changed ejection time t_(A) 'as well as a different cycle time t_(Z) '. In addition it is necessaryin that situation for a greater stroke movement than the referencestroke movement to be available for the annular piston within thestorage chamber, so that the amount of material to be stored, which isincreased in order to compensate for the deviating extruder deliveryoutput, can be accommodated in the storage chamber.

In regard to the second possibility which is identified by ", from thereference commencement of ejection (D), the material which is stored upto that moment in time (distance DB") is displaced from the storagechamber, at a reduced ejection speed. That will accordingly give alonger ejection time t_(A) ", within which correspondingly more material(distance F"C') is conveyed by the extruder into the extrusion head,which, together with the material V_(S) " which has admittedly beenstored, gives the required gross volume V_(B). A regulating procedure ofthat kind will be appropriate if a greater stroke movement on the partof the piston within the storage chamber is not available forcompensating for the delivery output. In addition intervention in thatway is possible only when the altered speed of discharge of the plasticmaterial from the outlet opening during the ejection operation does notresult in the preform being influenced in such a way as to suffer from adrop in quality. That regulating option also results in a change in thecycle time t_(Z) '.

Both of the possibilities shown in FIG. 6 will result in the occurrenceof portions by volume V_(S) ' and V_(E) ', and V_(S) " and V_(E) "respectively, which differ from the reference portions by volume V_(S)and V_(E) respectively and which together give the required gross volumeV_(B). The compensating operations result in a variation in the cycletime and the ejection time. The working procedure of the blow moldingmachine, which takes place in parallel relationship, must possibly beadjusted thereto. It will further be apparent that the illustratedregulating option results in an increase in the length of the cycle timeand thus only in a reduction in the level of productivity, although thepreform produced thereby satisfies the desired requirements.

FIGS. 7-10 therefore show further regulating options which in the idealcase do not result in variations in the cycle time, while furthermorethe required gross volume for a preform is ejected from the extrusionhead. In accordance with the structural factors described in theintroductory part hereof, it is generally conventional practice for thestroke movement to be performed by the annular piston and whichdetermines the volume of the material to be stored to be less than themaximum possible stroke movement which is predetermined by the storagechamber, so that the volume to be stored only fills up a part of themaximum volume of the available storage space. In many cases thearrangement of the positions for delimiting the piston stroke movementin the storage chamber will be such that both in the ejection movementand also in the movement of the piston during the filling phase, theannular piston can cover a longer distance than the reference distanceand can thus displace more material from the storage chamber than wasdelivered in the preceding storage phase, or more material may becollected in the storage chamber than is ejected. It is thereforepossible for an amount of material V_(K) to be ejected by the piston,which is not the same as the previously stored amount of material V_(S).That additional volume represents a compensating volume or a reservewhich can be used to compensate for deviations in the delivery output ofthe extruder.

FIG. 7 additionally shows on the ordinate a maximum volume V_(max) inrespect of the available storage space, within which the volume of thematerial to be stored is disposed.

Starting from an excessively low extruder delivery output, as shown inFIG. 7, the stroke movement to be performed by the annular piston in thesubsequent ejection operation is so set that the material displaced fromthe storage chamber is made up of the portion by volume which wasplasticised within the storage period in that cycle and which wasconveyed into the storage chamber, and the portion by volume which islacking at the end of the respective cycle, for the purposes ofachieving the required gross volume V_(B). The ejected volume for apreform is accordingly formed from the material stored in the cycle, thematerial delivered by the extruder during the ejection phase, and acompensating volume which is present in the storage chamber as areserve, for example from preceding cycles.

The amount of material to be compensated can be ascertained from theillustrated curve configuration. At the end of the cycle time t_(S)(moment in time E), a new preform is to be made available, with a grossvolume V_(B). According to the configuration of the delivery output(line AC') which is ascertained during the storage phase, in particularat the end thereof, the expectation is that, at the moment in time E,that is to say when all the preform is extruded, the extruder hasplasticised only a volume of material which corresponds to the distanceEC'. The volume of material which corresponds to the distance CC' islacking and must be compensated for.

Up to the predetermined moment in time D, only the amount of materialV_(S) ' (distance EF') was stored within the storage time t_(S), as aresult of the excessively low delivery output of the extruder. If theoperation of emptying the storage chamber begins at the predeterminedmoment in time D, the volume V_(K) ' to be ejected from the storagechamber must be increased relative to the volume V_(S) ' stored in thatcycle, by the amount missing in the delivery of the extruder, up to theend of the cycle (distance CC'), in order to obtain the full grossvolume V_(B) required.

At the moment in time D, the annular piston begins its ejection strokemovement in a position which corresponds to the point B'. When thepiston performs the reference stroke movement, it would eject the volumeV_(S), in which case the missing amount of plastic material V_(H) '(distance FF') must be taken from a reserve within the storage chamber.During the ejection phase the extruder plasticises the amount ofmaterial V_(E) ' (distance F'C') which, because of the deviation on thepart of the extruder, is also less than the reference amount V_(E). Theline parallel to the reference or desired delivery (line AC), throughthe point B', produces on the ordinate EC an intersection point C", thespacing of which from C' corresponds to the volume by which the deliveryof the extruder fell short, as a result of its excessively low deliveryoutput during the ejection phase. Therefore the reference volume V_(S)to be stored must be increased by the volume V_(A) ' corresponding tothe distance C'C", by suitably increasing the length of the strokemovement of the annular piston relative to the reference ejection strokemovement.

That portion of material must also be taken from the reserve in thestorage chamber. The sum of the missing material, which must be providedfrom the storage chamber, corresponds to the portions GE and FF'respectively, plus C'C" or CC'. Therefore the stroke movement of theannular piston does not terminate at a position which corresponds to thepoint E but at the moment in time E at the lower point G.

Because of the greater required stroke movement for the annular piston,an increase in the speed of the piston is required if the gross volumefor a preform is to issue from the opening of the extrusion head in thesame period of time (ejection time t_(A)). Therefore the curve portionB'G is of a somewhat steeper configuration than the curve portion BEconcerning the reference condition. The adapted speed of ejection of theannular piston provides at the same time that the appropriate speed ofdischarge for the plastic material is maintained, as the same amount ofplastic material issues from the extrusion head in the same period oftime.

FIGS. 8 and 9 which are described hereinafter, for the sake of enhancedcomprehension, show the deviation illustrated in FIG. 7 as well as thedeviating volumes V_(S) ' and V_(E) ' which would result from such adeviation.

FIG. 8 shows a regulating option in which the annular piston maintainsits speed corresponding to the reference condition (gradient of the lineBE) during the ejection phase. Because of the longer stroke movement, asa result of the volume V_(K) ' which is to be ejected from the storagechamber and which is increased by V_(L) ' (distance H'H"), there will bea longer ejection time t_(A) '. That increase in ejection time must betaken into consideration when arriving at the magnitude of the longerstroke movement and thus V_(L) ' as in that period of time materialcontinues to be plasticised by the extruder (distance K'H') and issuesfrom the extrusion head. Therefore, with the same deviation in respectof the delivery output, the compensating volume V_(L) ', as shown inFIG. 8, will be smaller than the compensating volume V_(A) ' as shown inFIG. 7, as a greater volume V_(E) " is passed into the extrusion head,in the increased ejection time. The total volume of material plasticisedby the extruder during the extended cycle time t_(Z) ' corresponds tothe distance E'H'. As a result of the longer cycle time and the strokemovement to be performed, which with an excessively low delivery outputis longer than the preceding stroke movement during the filling phase,the ejection stroke movement of the annular piston terminates at themoment in time E' at the lower position G'.

In a regulating situation as shown in FIG. 8, there will be a longercycle time t_(Z) '. However that may be avoided by a regulation effectas shown in FIG. 9, with an unchanged speed for the annular pistonduring the ejection operation. In FIG. 9, the ejection operation alreadybegins at an earlier moment in time D' which, together with an increasein the length of the stroke movement and thus an increase in the amountof material V_(K) " to be ejected, is so selected that, in the ejectiontime t_(A) " which is longer as a result, such an amount of materialV_(E) " is passed into the extrusion head from the extruder that thereference gross volume issues from the extrusion head. The total amountof material V_(B) ejected is accordingly made up of the material V_(S) "which was collected in the shortened storage time t_(S) ', the materialV_(E) " which was plasticised by the extruder in the extended ejectiontime t_(A) ", and the compensating volume V_(H) " which corresponds tothe insufficiently collected volume of material (distance F"F'), as wellas the compensating volume V_(M) ' which corresponds to the material(distance C'C") by which the output of the extruder fell short in theejection phase. In that connection the point C" corresponds to the pointof intersection of a line parallel to the reference or desired deliveryoutput line AC through the point B" with the ordinate EC. The volumesV_(S) " and V_(H) " again together give the reference volume V_(S) to bestored so that the annular piston only has to perform the referencestroke movement which is extended by a distance corresponding to thevolume V_(M) '. That regulating option can be carried into effect onlywhen the working procedure of the associated blow molding machine is notdisturbed by the earlier commencement of the ejection operation.

It is self-evident that the illustrated compensating operations can alsobe correspondingly transferred to those deviations which are caused byan excessively high level of delivery output by the extruder. Theexample in FIG. 10 shows a uniformly excessively high delivery outputfrom the extruder, corresponding to the line configuration AB'C'. At theend of the working cycle at the time E, the volume V_(B) for a preformis once again to be ejected. As a result of the delivery output beinghigher than the reference delivery output (line configuration ABC), atthe end of the cycle time the extruder has delivered a volume (distanceEC') which is greater than the gross volume V_(B) to be ejected for apreform.

The storage phase t_(S) involved the collection of a volume V_(S) 'which is greater than the reference volume V_(S) to be stored, by thevolume V_(H) ' (distance FF'). The nature of the delivery output whichis ascertained during the filling phase leads to the expectation thatthere is also an excess over the reference delivery in the ejectionphase, up to the moment in time E. The excess portion V_(A) ' which isto be related to the ejection phase accordingly corresponds to thedistance between the point C' and the point of intersection C" of a lineparallel to the reference delivery output line AC, through the point B',with the ordinate EC'.

If, at the moment in time D, the ejection operation begins from aposition of the annular piston which corresponds to the point B', thelength of the reference stroke movement must be reduced by a distancewhich corresponds to the volume V_(A) ' and which corresponds to thedistance C"C', so that the required gross volume can be ejected. Theamount of material which corresponds to the material V_(H) ' which wassupplied in excess in the storage phase t_(S) (distance BB'), togetherwith the material V_(A) ' which was supplied in excess during theejection phase t_(A) (distance C'C") must be temporarily put into thestorage chamber.

Accordingly the gross volume V_(B) to be ejected is made up of thematerial V_(E) ' (F'C') which is delivered by the extruder during theejection phase t_(A) and the amount V_(K) ' which is ejected from thestorage chamber by the annular piston and which is smaller than thereference volume V_(S) to be stored.

The result of the reduction in the length of the ejection strokemovement is that the speed of the piston must be reduced in order toattain the predetermined ejection time t_(A). Therefore the portion ofthe curve B'G is somewhat less steep than the configuration of theportion of the curve BE in respect of the reference condition. Theejection stroke movement of the annular piston therefore terminates atthe time E at the higher position G.

In regard to other regulating options, it will have to be noted that acompensating operation which is possibly required may result in ashorter cycle time and/or an earlier commencement of the ejectionoperation, which possibly cannot be adapted to the predeterminedprocedures in respect of time for the associated blow molding process,as for example the blow molding mold is still not ready to receive apreform at an earlier time of commencement of an ejection operation.

Theoretically the regulating options shown in FIGS. 7-10 are subjectedto limitations by virtue of the limits of the maximum stroke movement ofthe annular piston, if the compensating volume is completely filled whenthe delivery output of the extruder is excessively high, or if it iscompletely emptied when the delivery output is excessively low. Thelimits will generally be attained when the extruder has deviations inthe same direction, over a plurality of working cycles. At that momentthe delivery output must be changed in such a way that either the systemadopts the reference or desired condition or it is possible at leastagain to effect a compensating regulating procedure for example as shownin FIGS. 7-10. In that connection it may be provided that, for exampleif the delivery output of the extruder is excessively low, when acondition of complete emptying of the storage chamber is attained, theextruder may be brought to a stable operating condition which results ina slightly higher delivery output. After a plurality of cycles it willthen be necessary to set a slightly lower level of delivery output asthe storage chamber is then substantially filled. It may also bedesirable to fill up a completely emptied storage chamber by thecompensating volume of the storage chamber being filled again by atleast one cycle with an extended cycle time. Conversely, the completelyfilled compensating volume of the storage chamber could be at leastpartially emptied again, by performing at least one cycle with a reducedcycle time. In regard to the last-mentioned possibilities however it isalso necessary to take account of the cycle time of the associated blowmolding machine so that variations of that kind do not have an adverseeffect on the blow molding process. It will be particularly desirablefor such compensating operations for the compensating volume of thestorage chamber to be initiated before the invariable, structurallypredetermined abutment positions for the annular piston are attained, soas to ensure that a preform of the required gross volume is ejected ineach cycle. However the above-discussed possibilities will not benecessary in many cases as generally the delivery output of the extruderdoes not deviate exclusively in one direction from the referencedelivery output thereof.

The regulating procedures shown in FIGS. 6-10 have the advantage thatthe compensating operations to be carried out still give the desiredresult even when for example deviations in the delivery output of theextruder, which can also exceed the admissible tolerance range, aredetected just before the end of the filling phase, so that the effectsof an intervention in relation to the operating condition of theextruder could no longer be monitored during the same working cycle. Asthe adaptation operations shown in FIGS. 6-10 are primarily in respectof operating parameters for the subsequent ejection operation, theadaptation operations can be carried out virtually without any timedelay. In particular, these regulating procedures avoid frequentregulating intervention in relation to the operating condition of theextruder, in spite of which the gross volume required for the preform isejected with a high level of accuracy.

In particular therefore it will be desirable for the delivery conditionof the extruder, which is determined by the speed and the travel-timerelationship in respect of the annular piston during the filling phase,to be ascertained at least just before or at the beginning of theejection phase in order to obtain information about the delivery outputduring the ejection phase so that suitable interventions in respect ofthe operating parameters can possibly be carried out.

However, if there are excessive deviations in regard to the deliveryoutput of the extruder, it may also be desirable firstly to intervene inrelation to that operating condition, for example in accordance with theregulating system shown in FIG. 5, and, if that intervention does notgive the full desired result, to effect a supplemental regulationoperation as shown in FIGS. 6-10.

It is self-evident that regulation as shown in FIG. 7 or FIG. 10 canalso be applied to a deviation as shown in FIG. 4. In such a case itwould only be necessary for the lacking volume of material (distanceB'B) to be taken from the compensating volume of the storage chamber,being the volume by which the amount stored in the storage chamber fellshort during the filling phase t_(S), as the delivery output of theextruder has again reached its reference value. Accordingly anadditional volume V_(A) ' would not have to be ejected. Particularlywhen there is a disturbance of that kind in the operating condition, theadvantages of the control and regulating unit according to the inventionare apparent. As, in contrast to known regulating systems, the deliveryoutput of the extruder is monitored during the filling phase,compensation is only effected in respect of the missing stored material,because the reference delivery output which occurs again at the end ofthe filling operation is detected. That means that there is no erroneousintervention in relation to the operating condition of the extruder,which is otherwise usual in this situation, with the result that in thesubsequent working cycle the desired extruder output is maintained atany event when there are no additional influences occurring.

The description of the invention essentially relates to extrusion unitswhich comprise an extruder and with which a blow molding mold isassociated. It is self-evident that the invention can also be applied tothose extrusion units which have a corresponding number of extruders forexample for forming multi-layer preforms. If two or more blow moldingunits are associated with the extrusion unit, the invention enjoysparticular significance as the portion of the volume of material whichis conveyed into the extrusion head during the emptying phase isgreater, with the result that deviations in the delivery output of theextruder will have greater effects.

We claim:
 1. A process for the production of hollow bodies ofthermoplastic material by batch wise extrusion and blow molding ofhollow preforms comprising using an extrusion unit (2), which has atleast one continuously operated extruder (10), and an extrusion head (9)which includes at least one storage chamber (18) receiving materialplasticized in the at least one extruder and at least one member (19)reciprocatable with a stroke movement between two limit positions withinthe extrusion head to empty the at least one storage chamber, wherein aworking cycle of the extrusion unit has two operating phases, an initialfilling phase in which the at least one storage chamber is at leastpartially filled with displacement of the at least one member and aremaining ejecting phase in which the plasticized material in the atleast one storage chamber is advanced towards the outlet opening (20) ofthe extrusion head by means of the at least one member and a portion ofthe volume of the plasticized material is ejected from the outletopening to form at least one hollow preform, wherein the ejected portionof plasticized material by volume (V_(B)) is made up of a portion byvolume (V_(S)) which was previously stored in the at least one storagechamber and a portion by volume (V_(E)) which, during the ejectingphase, passed into the extrusion head from the at least one extruder,wherein at least once during the filling phase of a working cycle, avalue of one of actual speed and actual distance covered by the at leastone member is detected, the detected value is compared to apredetermined reference value for the at least one member, whichcorrespond to a reference delivery output of the at least one extruderand, at least when a deviation between the one detected value and thepredetermined reference value exceeds a predetermined referencedeviation value, at least one operating parameter of the extrusion unitis changed during a remainder of the one working cycle so that theactual volume of plasticized material (V_(B)) ejected by the at leastone extrusion head during the remainder of the one working cycle iscloser to a predetermined required gross volume for the hollow preformthan would have been ejected by the extrusion head in the absence of thechanging step.
 2. A process as set forth in claim 1 characterised inthat the reference delivery output of the at least one extruder isascertained from the predetermined volume (V_(B)) of the preform to beejected and the predetermined time of a working cycle (t_(Z)).
 3. Aprocess as set forth in claim 2 characterized in that the weight of theat least one hollow preform is obtained and compared to a predeterminedreference weight, wherein in dependence on a detected deviation, apredetermined value of ejected portion volume (V_(B)) is changed in thenext working cycle (t_(Z)) to define a reference delivery output for theat least one extruder.
 4. A process as set forth in claim 3characterizing that the weight measured is of at least one of thepreforms, which were produced in a plurality of successive workingcycles.
 5. A process as set forth in claim 3 further comprising thesteps of expanding at least a part of a central portion of the at leastone hollow preform into an expanded hollow body in a divided blowmolding mold having at least two mold portions which are movablerelative to one another and forming end portions of the at least onehollow preform into waste portions and characterized in that the weightof the least one preform is obtained by measuring the weight of theexpanded hollow body and all waste portions.
 6. A process as set forthin claim 1 characterised in that at least at the end of the fillingoperation (t_(S)) the speed of the at least one member is detected toascertain the actual delivery output of the at least one extruder (10).7. A process as set forth in claim 1 characterised in that at least atthe end of the filling operation (t_(S)) the distance covered and thespeed of the at least one member is detected for ascertaining the actualdelivery output of the at least one extruder (10).
 8. A process as setforth in claim 1 characterised in that at a plurality of moments in timeduring the filling operation (t_(S)) the distance covered by and/or thespeed of the at least one member (18) is detected for ascertaining theactual delivery output of the at least one extruder (10).
 9. A processas set forth in claim 1 characterised in that at least during a part ofthe filling operation (t_(S)) the distance covered by and/or the speedof the at least one member is continuously detected for ascertaining theactual delivery output of the at least one extruder (10).
 10. A processas set forth in claim 1 characterised in that a variation in the atleast one operating parameter is effected in dependence on the moment intime of detection of a deviation from a reference value.
 11. A processas set forth in claim 1 characterised in that a variation in the atleast one operating parameter is also effected during the working cycleof the extrusion unit, in which the deviation is detected.
 12. A processas set forth in claim 1 characterised in that a variation in the atleast one operating parameter is also effected in the same operatingphase in which the deviation is detected.
 13. A process as set forth inclaim 11 characterised in that a variation in the at least one operatingparameter is effected in the operating phase of the same working cycle,which phase follows the operating phase in which the deviation isdetected.
 14. A process as set forth in claim 1 characterised in that,upon a deviation in the actual delivery output from the referencedelivery output of the at least one extruder (10), the delivery outputthereof experiences a change only when the effects of the change can bemonitored in the same working cycle.
 15. A process as set forth in claim1 characterized in that upon detection of a variation in actual deliveryoutput from a predetermined reference delivery output of the at leastone extruder (10) in the filling phase of the one working cycle, strokemovement of the at least one member (19) is changed for the subsequentejection phase of the one working cycle in order to at least partiallycompensate for a difference resulting from the variation in amount ofplasticized material stored, wherein maximum stroke movement of the atleast one member within the at least one storage chamber is greater thana predetermined reference stroke movement of the at least one member.16. A process as set forth in claim 1 wherein a predetermined referencestroke movement of the at least one member corresponds to apredetermined reference volume of plasticized material to be stored inthe at least one storage chamber and wherein a ratio of thepredetermined reference volume to be stored to the volume of the ejectedportion is equal to a ratio of the difference between cycle time (t_(Z))and ejection time (t_(A)) to the cycle time (t_(Z)).
 17. A process asset forth in claim 1 characterized in that upon detection of a deviationin actual delivery output from a reference delivery output of the atleast one extruder (10) in the filling phase of one working cycle, speedof the at least one member is changed for the subsequent ejection phaseof the one working cycle such that the at least one preform is ejectedwith a volume (V_(B)) closer to a predetermined volume value for the onepreform than would have occurred had the speed of the one memberremained unchanged.
 18. A process as set forth in claim 1 whereinejection speed of the at least one member is ascertained from strokemovement performed during the ejecting phase.
 19. A process as set forthin claim 1 wherein upon detection of a deviation in actual deliveryoutput from a predetermined reference delivery output of the at leastone extruder, the extent of stroke movement and speed of the at leastone member is changed for the subsequent ejecting phase of the oneworking cycle in such a way that, on the one hand, a predeterminedvolume (V_(B)) for a preform is ejected from the extrusion head with agreater level of accuracy and, on the other hand, a reference ejectiontime (t_(A)) is maintained by the extrusion unit with a greater degreeof accuracy than would have occurred had the extent of stroke movementand speed of the one member remained unchanged.
 20. A process as setforth in claim 2 wherein length of the stroke movement of the at leastone member is changed with the change in length corresponding to anamount of plasticized material equal to a difference between apredetermined reference delivery amount of plasticized material and anactual delivery amount of plasticized material from the at least oneextruder during a predetermined reference ejecting phase time.
 21. Aprocess as set forth in claim 20 wherein an amount of material which issupplied to the extrusion head by the at least one extruder, by virtueof a change in the ejection time produced by the change in the length ofthe stroke movement when the ejection speed of the at least one memberis unchanged in relation to a predetermined reference ejection speed, istaken into consideration when changing the length of the strokemovement.
 22. A process as set forth in claim 17 wherein speed duringthe ejection phase of the at least one member is so changed thatejection time is altered thereby and such that an amount of plasticizedmaterial is conveyed into the extrusion head by the at least oneextruder during the altered ejection time, which amount corresponds to adifference between the predetermined volume value for the one preformand a previously stored amount of plasticized material.
 23. A process asset forth in claim 18 wherein a volume difference between an amount ofplasticized material ejected by the at least one member and apredetermined reference amount of plasticized material to be stored isset by means of altered length of the stroke movement of the at leastone member (19), and wherein the volume difference corresponds to theamount of plasticized material which is supplied to the extrusion head(9) as too much or too little, as a result of actual delivery output ofthe at least on extruder during the actual ejecting phase, which, due toaltered speed of the at least one member, at least approximatelycorresponds to a reference ejection time.
 24. A process as set forth inclaim 4 characterized in that the at least one hollow preform isexpanded under an increased internal pressure within a mold cavity of adivided blow molding having at least two mold portions moveable relativeto each other to form an expanded hollow body with waste portions andthe weight of the expanded hollow body is measured with its wasteportions to determine weight of the at least one hollow preform.